[VetteVet]

The white wire in the column plug is the brake power wire. the yellow wire is for the brake/turn signal on the left side . If the left turn signal works then the wiring is good on the yellow wire to the rear. The brake and turn signal use the same wire, yellow on the left and dark green on the right.

If both the brake and the turn signal work on the right then the dark green wiring is good and the problem is most likely in the turn signal switch for the left stop light contact.

IF you make a short jumper in the column connector between the white wire and the yellow wire then step on the brake pedal and the brake light comes on, then it confirms that the problem is in the turn signal switch.

It is not uncommon that the contacts inside the turn signal switch become dirty or corroded and fail to pass current through to the wires. Spraying some contact cleaner inside the switch on the contacts and maybe some WD-40 might be all that's needed.

Another check you can make is to swap the yellow and green wires in the column connector and see if the problem flips to the right side.